I've been using snap nocks for shooting 3 under for about 20 years.  I like the snap so I don't have to worry about the arrow falling off the string .  I shoot wood and aluminum I've never tried anything else.  I'm just wondering what others use and what is used on carbon arrows?

Depends on which carbon arrow you are talking about.There is your standard S-nock that will fit most carbons with a .246 ID.Like GT,Beman ICS,Blackhawk Vapor and some others along with aluminum with uni-bushing 20XX and larger.Then you have your Axis/MFX that take a X-nock with a .204 ID.Then your G-nock that usally require a bushing,they have a .166 ID.Then you have your H-nocks that are inbetween a X-nock and a S-nock.With your S-nocks they are a few different brands to pick from.For me my preferred S-nock or some call a Super nock I like Easton 3D Super nocks.The other nocks mentioned there are not many choices other the Easton.The G-nock though comes in two different throat sizes.A large groove with a .098 throat and a small groove with a .088 throat.

Must note that I am not well versed when it comes to any knowldge concerning the vast variety of available nocks.

However, have extensively utilized Bohning Double Lock Nocks on Beman ICS, Gold Tip and Victory VForce HV's with excellently satisfyng results.

Note: Have easily loosed over 100,000 arrows fitted with Bohning Double Lock Nocks. The need to personally search for another nock has not been warranted. Disclosing my personal satisfaction doesn't preclude the possibilities that better choices may be available.

I also shoot three under and have only used woods over the years. I am a fan of the Nirk nock. it has a snap in but I spend the time to either serve the string to just the right diameter or lightly file the nock so that the snap in is just right, not too heavy but doesn't pop out too easy. This has worked very well for me whether hunting or target shooting.

I use signature nocks on my CX's,but I open them up with fine sandpaper wrapped around a popcicle stick.Like wixwood said above,not to tight,but not falling off the string.
Leland